322 COMETS AND CENTAURS
Large Blackboard
Comet Wild 2
This small and unremarkable comet, discovered in 1978, is one that we know most about, since parts of it have been brought back to Earth. Wild 2 was discovered on its first passage through the inner solar system, several years after a close encounter with Jupiter’s gravity, which changed its orbit from a more or less circular one of 43 years’ duration to a more elliptical track with a
6.4-year period. This brings it within 1.6 AU of the Sun at its closest approach, and made it an ideal target for NASA’s Stardust mission, which visited the comet in 2004 and returned samples from its coma and tail in 2006. Laboratory analysis of Wild 2’s dust suggests that the comet formed in cold conditions at a great distance from the Sun, but incorporated dust from hotter regions, blown outwards on the solar wind. Subsequently, it spent most of its life in the cold outer reaches of the solar system before its orbit was disrupted to bring it closer to the planets. The dust also contained a wide variety of organic (carbon-based) chemicals, supporting the idea that comets might kick-start the development of life by delivering such chemicals to newly formed planets.
324 COMETS AND CENTAURS
Large Blackboard
Comet Churyumov- Gerasimenko
ften known by its catalogue number as Comet 67P, this small icy world is thought to have originated in the Kuiper Belt, but currently follows a 6.4-year orbit that takes it out to the orbit of Jupiter at aphelion, and brings it within 1.25 AU of the Sun at perihelion. It was the destination for the European Space Agency’s hugely successful Rosetta mission, which orbited 67P for two years from 2014 (throughout its perihelion passage) and placed a lander, called Philae, on its surface.
Structurally, Comet 67P consists of two distinct, irregularly shaped lobes, joined by a narrow ‘neck’. The different orientations of exposed layers within the lobes suggest that they began life as separate objects and, subsequently, stuck together after a low-energy collision. Rosetta’s surveillance revealed just how traumatic a comet’s perihelion passage can be; as ice escaped from beneath the surface, the probe recorded collapsing cliffs, boulders flung across hundreds of metres and large pits opening in the landscape.
O
326 COMETS AND CENTAURS
Blackboard label
Large Blackboard
Comet Encke
This curious comet was the second short-period comet to be identified, more than a century after the far more famous Comet Halley (see page 340). Encke’s orbital period
of just 3.3 years is one of the shortest known, taking it from the outer asteroid belt to within the orbit of Mercury. Comets ‘use up’ their reserves of ice with each perihelion passage,
yet Encke is still moderately active and so is thought to have arrived in its current orbit quite recently.
Encke leaves a trail of debris behind it that is thought to be
the source of the ‘Taurid’ meteor showers seen when Earth comes near the comet’s orbit in June/July and November. Close approaches between Earth and the comet itself happen every 33 years and, while it currently presents no danger, Encke’s frequent encounters with the strong gravity of the rocky planets result
in its orbit evolving over time. Some astronomers have even suggested that, in earlier times, Encke was much brighter and came closer to Earth, inspiring fear in our prehistoric ancestors.
328 COMETS AND CENTAURS
page 340
Comet
Shoemaker–Levy 9
Famous as the comet that smashed into Jupiter
with spectacular results in July 1994 (see page 210), Shoemaker–Levy 9 was discovered a little over a year earlier, already in orbit around the giant planet. It is thought to have been captured by Jupiter during a close encounter some time around 1970. The comet survived its capture intact, initially following an orbit with a roughly two-year period.
330 COMETS AND CENTAURS
page 210
A close encounter then brought it within 40,000 km (25,000 miles) of Jupiter’s cloud tops in July 1992. This is closer even than the innermost Jovian moon Metis, and well inside Jupiter’s ‘Roche limit’ – the region where the tidal forces exerted by the giant planet’s gravity become powerful enough to overcome the forces holding other large objects together. As a result, the cometary nucleus disintegrated into a series of fragments up to 2 km (1.2 miles) across, strung out into a ‘string of pearls’ as they made their final approach to Jupiter. Crater chains found on the surfaces of Callisto and Ganymede seem to suggest this is a common fate for comets captured by Jupiter’s gravity.
Comet Tempel 1
When first identified as a periodic comet in 1867, the faint short-period Tempel 1 followed a predictable 5.7-year orbit that took it from the outer asteroid belt to a perihelion just beyond Earth’s orbit and back. Astronomers lost track of
it in the late 19th century, however. It was presumed destroyed until 1967, when new research showed how it had been disrupted by close approaches to Jupiter in 1881 and, later, in the 1940s and 1950s. These interactions had first lengthened, then shortened the orbit – when subsequently rediscovered, it was found to have settled into a new 5.5-year pattern.
Tempel 1’s 14-km (9-mile) nucleus made it an ideal target for NASA’s Deep Impact probe, which encountered the comet in 2005, fired a barrel-like projectile into its surface and studied the fountain of material blasted into space by the impact. Analysis showed that Tempel 1’s material seems to have gone through a number of chemical changes – it was not the pristine relic from the early solar system that most had expected.
332 COMETS AND CENTAURS
Hartley 2
After releasing its impactor probe at Comet Tempel 1, NASA’s Deep Impact mission flew on to a rendezvous with another comet in November 2010. Roughly 1.1 km (0.7 miles) across and shaped rather like a bowling pin, Hartley 2 is the smallest comet so far to have been visited by a space probe. Orbiting the Sun in just under 6.5 years, it reaches aphelion beyond Jupiter, but comes with
0.05 AU of Earth’s orbit at perihelion.
Studies of Hartley 2 offered a possible solution to one mystery about Earth’s origins. While it was long assumed that much of
our planet’s present-day water originates from comets, close-up studies of comets themselves consistently found that comet ices contained too much ‘heavy’ water (made with deuterium, a heavier variant or isotope of hydrogen that is rare on Earth). Water vapour in Hartley 2’s coma, however, seems to match exactly with the characteristics of water on Earth. The difference in water content may be related to evidence that this comet formed closer to the Sun and in warmer conditions than its longer-period relatives.
334 COMETS AND CENTAURS
Large Blackboard
Chiron
Discovered in 1977, Chiron bears the telltale catalogue number of an asteroid, but was in fact the first object
in an entirely new class – the centaurs (see page 26). With a diameter of roughly 218 km (135 miles) according to the latest estimates, Chiron spends most of its 50-year orbit between those of Saturn and Uranus, though it crosses both around perihelion and aphelion.
At first, Chiron was thought to be a refugee from the asteroid belt – it was even given the asteroidal designation 2060 Chiron. Then, in 1989, astronomers noticed that it was growing steadily brighter and new photographs revealed the presence of a cometlike coma. This earned it the cometary designation
95P/Chiron, making it a rare object classed as both a comet and an asteroid. Unfortunately, little is known about Chiron’s physical properties, except that it has water ice on its surface, and that it is the largest in a subgroup of centaurs with dark, blue and grey surfaces (in contrast to Pholus, see page 338).
336 COMETS AND CENTAURS
page 26
page 338
Chiron
Uranus
Saturn
Jupiter
Pholus
The discovery of Pholus in 1992 led to the realization that Chiron was not alone, and that there are a significant number of centaur objects orbiting among the giant planets. Pholus bears the asteroid number 5145, and has a 92-year orbit that
is more elliptical than Chiron’s, going from inside the orbit of Saturn to outside that of Neptune, and back.
Physically, Pholus is slightly smaller than Chiron at about 180 km (112 miles) across. Its surface is brighter and distinctly red in colour, making it the prototype for the second major centaur subgroup. Centaurs seem to be either dark and blueish or bright and reddish, with no obvious continuum between the two types. Most experts agree that the redder ones carry significant amounts of carbon-based ‘organic’ chemicals, reddened by exposure to radiation from the Sun. But attempts to divine
the origin of the two categories have been frustrated by other shared characteristics; the red and blue centaurs mix in similar orbits and seem equally likely to show cometary activity.
338 COMETS AND CENTAURS
Pholus
Uranus
Neptune
Saturn
Jupiter
Comet Halley
The most famous comet of all, Halley was the first to have its periodic nature recognized. Edmond Halley realized that comets seen in 1531, 1607 and 1682 were, in fact, a single object, and predicted that it would return once again in 1658. Apparitions at more or less regular 76-year intervals have since been traced back to at least 240 BCE. The nature of its orbit suggests that Halley is one of the fairly uncommon family of comets that originate in the Oort Cloud but have their orbits shortened by interaction with the giant planets.
Halley’s most recent perihelion passage occurred in 1986, and although the geometry did not favour observers on Earth, an armada of spacecraft were sent to greet it. The European Space Agency’s Giotto probe flew within 600 km (373 miles) of the 16 × 8 km (10 × 5 mile) nucleus and returned images of a dark surface and violent jets. Dust blown off the nucleus but still following Halley’s orbit gives rise to two regular meteor showers on Earth, known as the Eta Aquariids and the Orionids.
340 COMETS AND CENTAURS
Comet Hale–Bopp
Thought to be the most widely observed comet of all time, Hale– Bopp was visible to skywatchers on Earth for 18 months. At its brightest, it outshone every star in the sky but one. Discovered in 1995 while it was still well beyond the orbit of Jupiter at a distance of 7.1 AU, early measurements suggested that it last visited the Sun 4,200 years ago. However, a close encounter with Jupiter in March 1996 saw its orbit change considerably and its next return will take a mere 2,380 years.
Hale–Bopp’s early brightness indicated a substantial nucleus, estimated at 60 km (37 miles) wide, although it could not be observed directly. Distinct jets emerging from the surface could be identified within the coma, allowing the comet’s rotation period to be calculated as 11 hours and 46 minutes. Some astronomers claimed the full pattern of Hale–Bopp’s activity could only be explained by the presence of a substantial satellite nucleus orbiting the main one, but this intriguing theory could not be confirmed before the comet retreated into the depths of space.
342 COMETS AND CENTAURS
Comet ISON
Comets are unpredictable by nature, and even those that promise to make brilliant apparitions as they round the Sun can ultimately disappoint. Comet ISON, for example, was expected to make a spectacular appearance as it passed Earth on its retreat from perihelion passage in November 2013, but never achieved its potential. The comet was discovered using a telescope from the International Scientific Optical Network (ISON) almost a year before perihelion, well outside the orbit of Jupiter, but already displaying a substantial coma. It approached on a hyperbolic trajectory – a path that would take it past the Sun just once before being flung into interstellar space – and was therefore either a fresh arrival from the Oort Cloud or, less likely, a visitor from beyond the solar system itself. However, as the comet passed within 1.17 million km (727,000 miles) of the Sun, it disintegrated. Most of the nucleus was destroyed, but a small fragment was later found, still continuing along its original trajectory with a greatly reduced brightness.
344 COMETS AND CENTAURS
Pluto
The brightest, and still the largest-known Kuiper Belt Object (KBO), with a diameter of some 2,377 km (1,477 miles), Pluto was discovered in 1930, and was historically considered a planet in its own right. Today, it is simply the largest dwarf planet (see page 16).
Circling the Sun every 248 years, Pluto’s elliptical orbit carries it between 29.7 and 49.3 AU from the Sun. At perihelion it is closer than Neptune, but its orbit is tilted at 17 degrees to the plane of the solar system and there is no risk of collision. As a result of its great distance and tiny size, it remained a mystery for decades. A giant moon, Charon (see page 350), was discovered in 1978, and four much smaller satellites have since been found. The orientation of their orbits reveals that Pluto’s axis of rotation is ‘tipped over’ at an angle of 120 degrees, giving it a Uranus-like pattern of extreme seasons. Other Earth-based observations confirmed the presence of a sparse nitrogen atmosphere, and a surface covered with nitrogen ice and other frozen gases.
346 THE KUIPER BELT AND BEYOND
page 16
page 350
Pluto’s surface
When New Horizons flew past Pluto in July 2015, it revealed an extraordinary world. Only one hemisphere could be mapped, since the planet’s southern half was in seasonal darkness, but the northern side alone proved to contain a surprising variety of terrains. A heart-shaped bright region consists of two distinct lobes across which the density of cratering suggests two different ages. The western lobe, known as Sputnik Planitia, is particularly young, suggesting Pluto has seen cryovolcanic activity in its recent past. The entire region is covered in a layer of nitrogen ice, with glacier-like features at the edges suggesting that this ice slowly flows across the surface. In contrast, Cthulhu Regio is a dark, heavily cratered landscape, probably unchanged for billions of years. Its brownish-red colour is thought to be due to tarry chemicals called tholins, created as solar wind particles trigger reactions between methane and nitrogen in the atmosphere. Elsewhere, mountains (most likely made of frozen water ice) rise up to 4 km (2.5 miles) high, perhaps somehow pushed up from a hypothetical ocean lying just beneath the surface.
348 THE KUIPER BELT AND BEYOND
Charon
Pluto has a remarkably complex system of moons for an object of its size. The largest, Charon, is just over half
the diameter of Pluto itself, and orbits in just 6.4 days at a distance of a mere 19,600 km (12,180 miles). As with many planetary satellites, tidal forces have slowed Charon’s rotation so that it keeps the same face permanently turned towards Pluto. In this case, however, the tides have done a similar job to Pluto, so that one hemisphere is locked facing Charon.
Charon is thought to have formed in a similar way to Earth’s moon, as debris thrown out from a collision with another large body coalesced in orbit around Pluto. The terrain is generally greyer than Pluto’s, and appears to be dominated by frozen water rather than the more volatile ices seen on its larger neighbour, but there is a prominent reddish region around
the north pole most likely covered in tholins. A mix of terrain types includes smooth plains, deep canyons and ice mountains, showing that Charon’s past has been just as active as Pluto’s.
350 THE KUIPER BELT AND BEYOND
Pluto’s moons
Beyond the orbit of Charon, Pluto has four other known satellites – Styx, Nix, Kerberos and Hydra (in order of distance from the planet). All four are elongated along one axis, with Hydra and Nix, at 55 and 42 km (34 and 26 miles) long respectively, considerably larger than Kerberos and Styx at 12 and 7 km (7.5 and 4 miles) long.
The orbits of these moons are almost perfectly circular and aligned with Pluto’s equator. This indicates that they are not captured bodies, but instead coalesced from a ring
of debris following the same major impact that created Charon. Hydra and Kerberos both have two very distinct lobes, suggesting
that they formed when a pair of smaller bodies collided, and the same is probably true of Styx and Nix. Orbit diameters range from 2.4 to 3.8 times larger than Charon’s, forming a surprisingly compact system. The tidal forces exerted by Pluto and Charon are continuously changing as a result, causing the moons to tumble along their orbits with a chaotic rotation period.
352 THE KUIPER BELT AND BEYOND
New Horizons’ most detailed views of Pluto’s outer moons to scale with Charon
Styx
Kerberos
Nix
Hydra
Charon
Albion
Although the existence of the Kuiper Belt was predicted as early as the 1940s (based on models of the solar system’s formation), it remained purely theoretical until 1992, when the first new object beyond Neptune since Pluto was discovered. Known for a long time by the designation 1992 QB1, it finally received an official name, 15760 Albion, in 2018.
With a diameter of around 140 km (87 miles), Albion is hard to study from Earth, but its orbit – and those of 2,400 other ‘Trans-Neptunian Objects’ now known – can clarify our picture of the outer solar system. Albion’s 289-year path around the Sun
is much less elliptical (ranging between 40.8 and 46.6 AU) and also less inclined than Pluto’s, tilted at just two degrees from the plane of the solar system. Such orbits are common in the ‘classical’ Kuiper Belt, and rather different from that of Pluto itself. Hence, astronomers divide the KBO population into ‘cold’ orderly objects like Albion (sometimes referred to as ‘cubewanos’, from ‘QB1’), and ‘hot’, more eccentric ones like Pluto (‘plutinos’).
Solar System in Minutes Page 13